Seconds to minutes after we're born, our umbilical cord gets snipped off, and the belly button begins.
Belly buttons usually start out as "outies," and most pop in to form "innies." Only 10% of people have outies.
Sources: AMNH, Le Bonheur Children's Hospital
Sometimes, an expanding belly can turn a pregnant woman's innie into an outie.
Source: Womenshealth.gov, Everyday Health
Researchers determined the "perfect navel" was a small, "T- or vertically shaped umbilicus with superior hooding," as shown. This is obviously subjective.
"Umbilical protrusion (an outie) was unappealing," the researchers wrote in the journal Plastic and Reconstructive Surgery.
A couple thousand people get belly button plastic surgery every year. And many of the 180,000 people who get tummy tucks per year have their navels touched up, too.
Plastic surgeons can do boob jobs through the belly button. The only scar is in the belly button, so there are no marks on the breasts like with traditional augmentation.
Belly button lint comes from our clothes. Men tend to have more because it gets caught in their belly hairs.
Sources: The Bellybutton Lint Survey, Quora
Researchers studied 60 people's belly buttons and found almost 2,400 different species of bacteria.
Source: PLOS ONE
All mammals that have a placenta have belly buttons, too — one more commonality to unite us all.
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Source: University College London
Further reading: There’s No Proof Taylor Swift Has a Belly Button, Taylor Swift's belly button